Air bag systems have become a standard vehicle safety feature to prevent injury to vehicle occupants. Unfortunately, in certain circumstances, early first generation air bags sometimes caused injury to the vehicle occupants due to the indiscriminate nature in which the air bags were inflated. The air bags would inflate at maximum force regardless as to whether the occupant was a child or an adult, whether the occupant was properly seated to face the air bag, or whether the occupant was too close to the point of air bag deployment. The application of maximum deployment force of the air bags to children or adults of small stature has resulted in injury even in relatively low speed collisions.
In view of the problems associated with the first generation air bag systems, a variety of "smart" air bag systems have been developed in an attempt to prevent unwanted injuries from occurring due to air bag deployment. These second generation air bag systems include sensors for detecting the presence of an occupant within a vehicle. U.S. Pat. No. 5,906,393 issued to Mazur et al., for example, discloses a system in which a weight sensor is used to determine the presence of an occupant in a vehicle seat. Other systems have been developed to specifically detect the presence of a child seat. U.S. Pat. No. 5,901,978 issued to Breed et al., for example, discloses a system for detecting the presence of a child seat that utilizes ultrasonic transducers.
While the above-described systems are improvements over the first generation systems, they are generally limited in the amount of information they can provide to control air bag deployment. It would be preferable to provide a system that could detect not only the presence of an occupant, but also distance of the occupant from the air bag and the occupant's angular direction and angular extent. For example, as children are generally narrower in width than adults, it would be beneficial to provide some measure of the angular extent of the occupant to provide a simple method of determining if the occupant is a child or an adult of small stature.
Ultrasonic or acoustic range finding in itself has been applied in many applications including, for example, lens focusing systems for in cameras in which an ultrasonic range finder computes the distance to an object and adjusts the lens focus accordingly. In such acoustic range finding applications, an appropriate transducer generates an acoustic signal as a short duration pulse. The pulse is reflected off of nearby objects and is received by the same, or another, transducer. As the speed of sound in air is a known quantity, the distance of the object from the transducers can be calculated from the transit time of the acoustic pulse.
Ultrasonic range finders typically use ultrasonic frequencies which are inaudible to the human ear. These high frequencies have inherently shorter wavelengths, which lead to greater positional accuracy than audible frequencies. Some systems known in the art use several simultaneous signals with differing frequencies. These simultaneous signals are generated to provide at least one readable signal in the presence of acoustic interference.
Ultrasonic sensors are typically made from a single transmitter/receiver transducer. A brief ultrasonic pulse is transmitted, and this is reflected from a nearby object. The transducer, now used as a receiver, detects the reflected pulse. This type of sensor will give object distance information, but provides no angular position or extent information.
Phased array radar systems utilize a stationary array of transducers to generate object distance, angular extent, and angular position information. An array of many transducers driven at different amplitudes and phases can produce a lobe pattern of one narrow beam which is steerable over a wide angle. This technique is called aperture synthesis and it is used in phased array radar systems.
The beam is formed by the interference of the radar waves, a consequence of the principal of linear superposition. In linear superposition, the radiation of one source combines with that of another source to either increase or decrease the radiation amplitude at a point, causing constructive interference or destructive interference respectively. A well known example of this principal is the Young Experiment of 1802 in which light is passed through a pinhole to create a point light source, then it is through two other pin holes, finally the light is projected on a screen. A regular pattern of light and dark bands appears on the screen, which is caused by the interference of the two point sources. A more detailed analysis of the Young Experiment appears in D. Halliday and R. Resnick, "Physics for Students of Science and Engineering," Part II, Second Edition, John Wiley & Sons, Inc., New York, 1962, pp. 976-982. Further information may be found in Grant R. Fowles, "Introduction to Modern Optics," Holt, Rinehart and Winston, Inc., New York, 1968, pp. 62-66. The linear superposition effect is applicable to light waves, radar waves, and acoustic waves.
A similar steered beam system which uses a stationary array of transducers would be desirable for vehicle occupant detection. However, due to the inherent characteristics of radar wavelength and frequency, radar is not accurate enough for close range use in measuring the relatively small variations in distance between a passenger and an automotive air bag. Therefore, a device which uses aperture synthesis technology and facilitates accurate short range distance measurement is needed.
If an array of acoustic transducers were utilized, an interference pattern could be formed if the transducer spacing is about the same as the wavelength of the acoustic signal. If the speed of sound in air is about 344 m/sec, an ultrasonic transducer operating at a frequency of 68.8 kHz will have a wavelength of 5 mm, while higher frequencies will have proportionately smaller wavelengths. If two transducers are used and spaced apart by a wavelength, the transmitted beam pattern will be similar to the beam pattern shown in FIG. 1 in which a central main lobe is produced with corresponding side lobes. If a phase shift from 0-180 degrees is introduced between the transducers, the main lobe is steered to one side and the intensity of the side lobes is changed until a symmetric lobe pattern is achieved at a 180 degree phase shift as shown in FIG. 2. Thus, a simple two transducer system could offer limited scanning capability and the ability to distinguish between objects at the front and sides of the transducers. A larger array of transducers, for example a 4.times.4 array, driven at different amplitudes and phases could produce a lobe pattern including one narrow beam steerable over a wide angle as illustrated in FIG. 3.
While efficient ultrasonic transducers are available commercially, these transducers are typically formed as a resonant diaphragm with diameter of about one wavelength since this structure produces an efficient conversion of electrical energy to sound energy. The speed of sound in a typical diaphragm material is much faster than the speed of sound in air. As a consequence, the wavelength in the diaphragm is larger than that in air at the same frequency, and the diameter of these diaphragm transducers is larger than one wavelength in air. Thus, the closest possible spacing of this type of transducer exceeds one wavelength in air.
The inherent diameter of the diaphragm transducers presents a problem when constructing an array of acoustic sources to generate a steered radiant beam. In the simple case of two sources separated by a spacing d and driven in phase by the same excitation voltage, the angular separation in radians between maxima or minima of the interference pattern is about .lambda./d. For a transducer separation of twice the wavelength, the resultant interference pattern has a beam perpendicular to the plane of the transducers defined by minima at 14.3 degrees at either side of the center. A pair of secondary side lobe beams are formed at 28.6 degrees on either side of the central beam. If it is desired to probe only the region directly in front of and perpendicular to the plane of the transducer, there will be undesirable interfering signals from these secondary side lobe beams and additional side lobe beams at larger angles. FIG. 4. illustrates angle spacing to the first minimum as a function of separation of two acoustic sources.
As described above, a desirable method of producing a steered acoustic beams is to use an array of many transducers driven at different signal phases. The beams from the individual transducers will supplement each other to give a large signal amplitude in a given direction. Destructive interference between the beams will lead to a small signal amplitude away from the chosen direction. In the two transducer case, however, the presence of side lobes will lead to ambiguous signals. In range finding applications, accurate positional information will be lost since acoustic reflections would be generated by the side lobes as well as the main beam. A solution to the side lobe beam problem would be to place the array transducers very close to each other, preferably less than one wavelength. Unfortunately this is impossible for the aforementioned resonant diaphragm type of transducer, due to the inherent diameter of the transducer which is larger than one wavelength.
In view of the above, it is an object of the invention to provide an apparatus and method that utilizes a steered acoustic beam for object location and classification, and further to incorporate such apparatus and method into a detection system for detecting the presence of an object within a compartment of a vehicle, as well as the objects's distance, angular direction and angular extent.